1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device and a contactless IC card and, in particular, to a technique suitable for a contactless IC card (hereinafter, simply referred to as an “IC card”) and a receiver circuit of a semiconductor integrated circuit device that is mounted on the IC card.
2. Description of Related Art
A so-called IC card in which a semiconductor integrated circuit device and an antenna are mounted exchanges information with an interrogator and its semiconductor integrated circuit device and implements various functions such as transmitting data retained by it and storing data transmitted from the interrogator.
The semiconductor integrated circuit device mounted on the IC card receives an RF signal radioed from the interrogator by the antenna mounted on the contactless IC card, rectifies and smoothes the voltage produced across the antenna, and generates an internal voltage that is required for internal circuit operation.
Recently, demand for advanced functions of the IC card has been increased. For instance, a dual-type IC card with a contactless interface and terminals for input and output is typical of such advancement. With the advanced functions of the IC card, the consumption current of the semiconductor integrated circuit device mounted on the IC card increases. In consequence, it is becoming difficult to extend the communication distance between the IC card and the interrogator in operation via the contactless interface.
Meanwhile, in data transmission from the interrogator to the contactless IC card, an amplitude shift modulation scheme, so-called Amplitude Shift Keying (ASK) that modulates the amplitude of the RF signal is used, typically, in compliance with the ISO 14443 standard.
Conventionally, a demodulator for this kind of contactless chip card having the following structure is known: it is made up of a detector circuit including an antenna, a rectifying circuit which rectifies a detected signal, a bandpass filter which generates a low frequency signal and a high frequency signal from the rectified signal, a comparator stage which compares the low frequency signal and the high frequency signal applied to it, and a memory which stores the result of the comparison by the comparator stage (e.g., see patent document 1).
A comparator (Schmitt circuit) having hysteresis characteristics for use in the modulator is known and this circuitry is configured such that it detects a changing point on the envelope curve of a receive signal, determines a positive or negative direction of change occurring at the changing point, and sets the operating point higher than a reference potential by a predetermined potential for detecting the potential change of a signal that is applied to input and output terminals when data demodulation is performed (e.g., see patent document 2).
[Patent document 1] Japanese Patent No. 3177502
[Patent document 2] Japanese Patent Laid-Open No. H11-214960